Method for hot-dip finishing

ABSTRACT

The invention relates to a method and a device for coating the surfaces of, in particular, strip-like material, for example, a non-ferrous metal strip or a steel strip, with at least one metallic coating by running through at least one container filled with the liquid melt coating material. The metal strip for coating runs through the molten bath of coating material within the container from bottom to top, suitable guide means are provided for the above. The sealing of the container base is achieved by means of circumferential permanent magnets.

The invention concerns a method for coating the surface of a product, especially a strip-shaped product, for example, nonferrous metal strip or steel strip, with at least one metal coating by passing the product through at least one molten metal bath space that contains the molten coating material. The invention also concerns a device for carrying out the method.

In conventional hot dip coating of strip (referred to here as Method 1) with Zn, Zn—Al, Al, or Al—Si alloys, the strip runs in the coating section from an annealing furnace under conditions of air exclusion into the molten metal and is deflected vertically and stabilized by various arrangements of nondriven rollers (see FIG. 1). This applies to all of the specified coating metals/alloys used in hot dip coating.

A disadvantage of Method 1 is that the rollers and the bearings of the rollers are located within the molten material, and all parts are exposed to chemical attack by the molten material. The service life of the parts that are used within the molten material is limited. In addition, a large volume of molten material with a correspondingly large dip bath is necessary to accommodate the rollers and all of the bath equipment. 200 to 400 t of molten zinc are customary in hot dip galvanizing. Due to this large volume, rapid regulation of the temperature and alloy composition of the melt is not possible. Large fluctuations of the specified parameters must be accepted and sometimes result in loss of quality, since measures related to the production of the alloy and those related to influencing the strip quality are carried out in the same tank and thus affect one another.

Another disadvantage is that the production speed cannot be increased to realize an economical plant output (about 180 m/min), especially in the case of thin strip <0.5 mm. One reason for this is that relative motion can develop between the rollers located in the bath and the strip. If the tension is increased in an effort to avoid this problem, there is the risk of strip breakage. This results in scrap and prolonged plant shutdowns.

The jet stripping system located above the zinc hot dip bath further limits the maximum strip advance speed of a hot dip galvanizing installation (see FIG. 1). The coating thickness is adjusted by air or nitrogen, and the minimum coating thickness that can be produced increases with increasing strip speed. This means that thin coatings cannot be produced at high strip speeds. However, certain demanding applications require thin coatings (<25 g/m² on one side in hot dip galvanized sheet).

So-called vertical hot dip galvanizing is well known as an advanced method for the hot dip coating of ferritic steel strip made of soft unalloyed steels and is described in various patents, such as EP 0 630 421 B1, EP 0 630 420 B1, and EP 0 673 444 B1.

In this method (referred to here as Method 2), the strip passes from bottom to top through a working tank filled with molten metal composed of zinc and/or Al alloys after it has been subjected to a heat treatment. The strip enters the molten bath under conditions of air exclusion. The volume of molten metal (about 2-5 t of molten zinc) is much smaller than in Method 1. The qualitative problems described above also do not occur, since the measures related to the production of the alloy are carried out in a reservoir located alongside the line, while measures to influence the strip quality are carried out separately in the working tank.

The working tank and the furnace chamber located below it are connected by a gastight ceramic duct, which is about 800 mm high and has a passage width for the strip of only a maximum of 20 mm. The working tank is sealed at the bottom to prevent molten metal from flowing down into the furnace chamber by means of a seal produced within this duct by two inductors arranged at the side of the duct or strip. These inductors induce an electromagnetic traveling field, which produces an upwardly directed force that prevents the molten metal from flowing down. This inductive system acts like a pump, so that exchange of the melt in the duct is ensured.

Method 2 is characterized by the fact that, at least in the coating area up to the hot dip bath, significantly higher strip speeds on the order of 300 m/min can be realized even with thin steel strip, since there are no rollers in the coating tank.

After the strip has passed through the coating unit from bottom to top at a temperature, e.g., in the case of hot dip galvanizing, of about 460° C., the desired thickness of the metal coating is adjusted a short distance above the hot dip bath by the jet stripping process, as in Method 1. This process is comparable to the process used in Method 1 and involves the blowing of compressed air or nitrogen.

As in Method 1, the jet stripping process in Method 2 also limits the maximum possible strip speed when thin coatings are being applied. However, Method 2 offers greater degrees of freedom for the galvanizing parameters of melt temperature and viscosity and alloy composition, which likewise affect the coating thickness. For this reason, it is to be expected that a higher strip speed can be selected in Method 2 than in Method 1 for the same coating thickness. In contrast to Method 1, Method 2 has not yet been tested on the industrial scale. So far only pilot plant trials with narrow strip have been conducted. These trials were successful.

However, an obstacle to an increase in speed is presented by the fact that the strip subsequently must be cooled below 300° C. in the upwardly traveling strand before the first deflection. If the temperature is higher, there is the danger that metallic particles will grow on the first contact roller or deflecting roller in the cooling tower and cause irreparable surface defects on the material.

The cooling is usually produced by several successive air cooling lines. However, the cooling effect or, more precisely, the cooling rate, is limited by the medium and cannot be increased at will on a fixed length of line (e.g., two times 15 m) with the use of air as the cooling medium. With increasing strip speed or with increasing mass throughput, the cooling lines must be lengthened. However, it then becomes necessary to raise the upper deflecting roller in the cooling tower of a hot dip coating installation.

In installations that are operated by Method 1, the height of the upper deflecting roller is usually 30-60 m. In the case of Method 2, it would be necessary, at high strip speeds, to lengthen the cooling lines accordingly, and the height of the cooling tower would have to be increased to about 80-90 m. This requires higher capital expenditures for buildings and foundations.

The free-running, unstabilized strip length in the tower would thus increase, and the strip flow would be destabilized, so that vibrations may occur, and the product quality may be adversely affected. The use of other cooling media in the upwardly traveling strand is problematic, and so far this problem has not been solved on the industrial scale.

Another problem, which concerns the electromagnetic seal used in Method 2, is that the forces that act on the liquid melt also act on the ferritic strip. Undesired contact of the strip with the duct due to the magnetic forces of the sealing inductors is possible only by additional expensive measures. This requires additional stabilizing coils and expensive automatic control technology.

The objective of the present invention is to avoid the specified disadvantages of Methods 1 and 2 and to create a high-speed hot dip coating installation without a cooling tower, which combines the least possible construction expense with optimized capital investment costs and high plant output with the best production quality.

This objective is achieved with a method of the type described in the introductory clause of claim 1 by sealing the molten metal bath space by means of rotating permanent magnets. The sealing of the molten metal bath space by rotating permanent magnets is considerably more reliable and less expensive than an electromagnetic solution, and significantly less power is needed for the rotation than for an electromagnetic seal, which is an advantage especially in the event of a power failure.

Refinements of the method are described in the dependent claims. A device and refinements of this device for carrying out the method of the invention are the objects of additional claims.

The invention is described below with reference to several embodiments shown schematically in the drawings.

FIG. 1 shows a conventional strip coating method.

FIG. 2 shows an advanced coating method in accordance with the state of the art.

FIG. 3 shows the coating method of the invention and a correspondingly designed high-speed hot dip coating installation in operation.

FIG. 4 shows the installation in FIG. 3 in a start-up situation.

FIG. 5 shows the installation in FIG. 3 during shutdown after operation.

In accordance with FIG. 3, after a deflection in the furnace under conditions of air exclusion, strip 1 runs vertically downward into a molten metal bath space that contains the hot dip bath. This hot dip bath is sealed towards the bottom. This requires forces, but these forces are not electromagnetic in nature, but rather are produced by rotating permanent magnets. The sealing of the melt with permanent magnets is well known in itself, but the prior art involved the use of rectangular ducts. A duct shape like this cannot be changed with respect to clearance and shape.

By contrast, the present invention proposes two adjacent rotors 5, 5′. The rotors are tubes 6, 6′ made of materials that are resistant to heat and molten metal, preferably ceramic materials. Rollers, on whose cylindrical surface permanent magnets 4 are mounted, rotate inside these tubes 6, 6′, whose diameters may be freely selected. The rotors 5, 5′ can be adjusted to the melt or to the strip. It is also possible to close the gap 7 when the installation is shut down or is being started up.

Permanent magnets are significantly less expensive than electromagnetic sealing by means of coils or inductors, and much less power is required for the rotation than for an electromagnetic seal, which is an advantage especially in the event of a power failure.

In addition, much higher field strengths can be produced with permanent magnets (by a factor of 3) than by the electromagnetic method. These high field strengths and the resulting higher forces are needed for the stripping process for adjusting the desired coating thickness on the steel strip. In the previously known methods, this adjustment must be accomplished by additional stripping jets.

Additional measures within the magnetic seal and stripping are no longer required in the method of the invention, since the region of the narrowest passage of the strip 1 through the sealing unit is now only a few millimeters. Furthermore, the strip can be supported at much shorter lengths than in the previously known Methods 1 and 2, since the strip 1 can be immediately cooled and deflected into a water bath 9 directly below the sealing unit. The support length in the present invention is preferably only about 5,000 mm, whereas in Method 1 it is about 8-10 times greater, and in Method 2 it is greater still.

Another advantage of the method of the invention is that the surface of the molten metal, preferably the molten zinc, in the coating area is within a protective gas atmosphere, which preferably consists of a nitrogen/hydrogen mixture, so that interfering oxidation of the molten zinc cannot occur. In the previously known Methods 1 and 2, this can be accomplished only with considerable additional expense. Furthermore, in the previous methods, it is necessary for the surface of the zinc bath to be accessible for certain types of manual work. In the present invention, access to the surface of the hot dip bath for the purpose of removing particles of oxidized metal is unnecessary.

In the embodiment in FIG. 3, the installation for the hot dip coating of a nonferrous metal strip or a steel strip 1 is shown in continuous operation.

The incoming strip 1 to be coated passes through a tension roller 17 and then through a lock 18, which hermetically seals the protective gas atmosphere prevailing inside the hot dip coating installation from the ambient, oxygen-containing atmosphere.

In the galvanizing chamber 14 which follows, the strip 1 is vertically deflected by guide rollers 13 towards the coating section 19. Upon entering the coating station 19, the strip 1 passes vertically from top to bottom through the bath of molten metal 3 maintained in the gap 7 between the rotors 5, 5′ and thus receives the desired coating.

At the lower end of this hot dip bath 3, in a gap formed between spaced rotors 5, 5′, the molten metal is prevented from running out at the bottom by magnetic forces of magnetic fields or traveling magnetic fields of the rotating permanent magnets 4. The rotors 5, 5′ are located inside the tubes 6, 6′ that surround them. The coating station 19 is surrounded on the outside by a duct-like housing and holds the rotors 5, 5, which are spaced a variable distance apart. They are surrounded by the tubes 6, 6′, which are made of materials that are resistant to heat and molten metal, especially nonmagnetic materials and preferably ceramic materials.

The permanent magnets 4 rotate inside these tubes 6, 6′.

The molten metal required for coating, which must be continuously replenished, is conveyed in controlled amounts by a metal pump 12 from a reservoir 8, in which it is conditioned, into the gap 7 between the rotors 5, 5′. The strip 1, which is coated in the gap 7, passes through the gap at the lower end and then passes in succession through an arrangement 15 for air stabilization and an arrangement 16 for water cooling.

After it has passed through the water bath 9 and a tension roller 20, it is removed from the installation for further use or treatment.

The additional FIGS. 4 and 5 show the method of the invention

(a) in a start-up situation, and

(b) during shutdown after operation.

(a) Start-Up Situation:

-   -   strip not running     -   rotors rotate gap between rotors closed     -   melt is supplied     -   furnace chamber closed.

(b) During Shutdown after Operation:

-   -   return of melt     -   rotors rotate     -   gap closed     -   furnace chamber opened.         FIG. 1: Method 1.

-   KEY: Kühlung=cooling

-   Ofen=furnace

-   Band=strip

-   Abstreifdüsen=stripping jets

-   SchmelzgefäB mit flüssigem Metal=hot dip coating tank with molten     metal

-   Schmelze=melt

-   Umlenk-/Stabilisierungsrollen=deflecting/stabilizing rollers     FIG. 2: Method 2.

-   KEY: Kühlung=cooling

-   Ofen=furnace

-   Nachglüh-Ofen=reannealing furnace

-   Abstreifdüsen=stripping jets

-   SchmelzgefäB mit Induktionskanal=hot dip coating tank with induction     duct

-   Ofengehäuse=furnace housing

-   Umlenk-/Stabilisierungsrollen=deflecting/stabilizing rollers     FIG. 3: Method in accordance with the invention.

-   KEY: Ofen=furnace

-   Spannrolle=tension roller

-   Schleuse=lock

-   Verzinkungskammer=galvanizing chamber

-   konstanter Betrieb—Band läuft=constant operation—strip is running

-   Rotoren drehen=rotors are rotating

-   Spalt geöffnet=gap open

-   Ofenkammer geschlossen=furnace chamber closed

-   Schmelze=melt

-   Spalt=gap

-   Rotoren mit Permanentmagneten=rotors with permanent magnets

-   Beschichtungssektion=coating section

-   Keramikrohre=ceramic tubes

-   Schmelze=melt

-   Pumpe=pump

-   Vorlagebehälter=reservoir

-   Luftstabilisierung=air stabilization

-   Wasserkühlung=water cooling

-   Wasserbad=water bath

-   Spannrolle=tension roller     FIG. 4: Method in accordance with the invention.

-   KEY: Anfahrsituation—Band läuft=start-up situation—strip is running

-   Rotoren drehen=rotors are rotating

-   Spalt geschlossen=gap closed

-   Schmelze wird zugeführt=melt is fed in

-   Ofenkammer geschlossen=furnace chamber closed     Otherwise see FIG. 3, i.e.:

-   KEY: Ofen=furnace

-   Spannrolle=tension roller

-   Schleuse=lock

-   Verzinkungskammer=galvanizing chamber

-   konstanter Betrieb—Band läuft=constant operation—strip is running

-   Rotoren drehen=rotors are rotating

-   Spalt geöffnet=gap open

-   Ofenkammer geschlossen=furnace chamber closed

-   Schmelze=melt

-   spalt=gap

-   Rotoren mit Permaneptmagneten=rotors with permanent magnets

-   Beschichtungssektion=coating section

-   Keramikrohre=ceramic tubes

-   Schmelze=melt

-   Pumpe=pump

-   Vorlagebehälter=reservoir

-   Luftstabilisierung=air stabilization

-   Wasserkühlung=water cooling

-   Wasserbad=water bath

-   Spannrolle=tension roller     FIG. 5: Method in accordance with the invention.

-   KEY: Stillstand nach Betrieb=shutdown after operation

-   Rücklauf der Schmelze=return of the melt

-   Rotoren drehen=rotors are rotating

-   Spalt geschlossen=gap closed

-   Ofenkammer geöffnet=furnace chamber open     Otherwise see FIG. 3, i.e.:

-   KEY: Ofen=furnace

-   Spannrolle=tension roller

-   Schleuse=lock

-   Verzinkungskammer=galvanizing chamber

-   konstanter Betrieb—Band läuft=constant operation—strip is running

-   Rotoren drehen=rotors are rotating

-   Spalt geöffnet=gap open

-   Ofenkammer geschlossen=furnace chamber closed

-   Schmelze=melt

-   Spalt=gap

-   Rotoren mit Permanentmagneten=rotors with permanent magnets

-   Beschichtungssektion=coating section

-   Keramikrohre=ceramic tubes

-   Schmelze=melt

-   Pumpe=pump

-   Vorlagebehälter=reservoir

-   Luftstabilisierung=air stabilization

-   Wasserkühlung=water cooling

-   Wasserbad=water bath

-   Spannrolle=tension roller 

1. Method for coating the surface of a product, especially a strip-shaped product, for example, nonferrous metal strip or steel strip (1), with at least one metal coating by passing the product through at least one molten metal bath space that contains the molten coating material (3), in which method, the molten coating material is fed from a reservoir (8) into a gap (7) between two counterrotating rotors (5, 5′), and the strip (1) is conveyed from top to bottom through the melt (3) and between the rotors (5, 5′), wherein the bottom of the gap (7) is sealed by rotating permanent magnets, and rotating rollers (4, 4′), in which the permanent magnets are incorporated, are installed inside the rotors (5, 5′).
 2. Method in accordance with claim 1, wherein the rotors (5, 5′) are formed from materials that are resistant to heat and molten metal, especially nonmagnetic materials, preferably with the use of ceramic tubes (6, 6′).
 3. Method in accordance with claim 1, wherein the molten metal (3) is conveyed in controlled amounts by a metal pump (12) from a reservoir (8) into the gap (7).
 4. Method in accordance with claim 1, wherein the rotating permanent magnets simultaneously serve to adjust the desired thickness of the coating on the metal strip (1).
 5. Method in accordance with claim 1, wherein, after it has been deflected in the preheating furnace (2) under conditions of air exclusion, preferably in a protective gas atmosphere, the metal strip (1) is conveyed vertically downward through the molten metal (3).
 6. Method in accordance with claim 1, wherein the coated strip (1) is air-stabilized and/or water-cooled as short a distance as possible below the seal of the molten metal bath space or the molten metal (3) or the rotors (5, 5′).
 7. Device for carrying out the method in accordance with claim 1, which comprises at least one molten metal bath space for holding a molten coating material for strip-shaped metal products (1), with the formation of a gap (7) between two counterrotating rotors (5, 5′), which seal the gap at the bottom, wherein rotating rollers (4, 4′), on whose cylindrical surface permanent magnets are mounted, are installed inside the rotors (5, 5′).
 8. Device in accordance with claim 7, wherein the molten metal bath space that holds the hot dip bath (3) is formed by the upper central space between the rotors (5, 5′).
 9. (Currently Amended) Device in accordance with claim 7, wherein at least the rotors (5, 5′) are surrounded by a housing to form a protective gas atmosphere.
 10. Device in accordance with claim 7, wherein the rotor housing is connected with an upper chamber (14) for the purpose of feeding the metal strip (1) to the rotor housing from above, with a reservoir (8) for molten metal, with arrangements for air stabilization (15) and water cooling (16) of the strip (1) installed below the rotor housing, and possibly with another water bath (9). 